This invention relates in general to compressors and in particular to a new and useful turbocompressor having an improved control for preventing pumping.
The invention relates to a method of controlling turbocompressors which is particularly well suited for the so-called pumping limit control to prevent the pumping of a compressor. But it can also be applied to other controls such as an antichoke control, turbine speed control, etc.
A method for the pumping limit control is known from the article "Turbolog--The Electronic Control System for GHH Turbomachiens" in Nachrichten fur den Maschinenbau (News for Machine Builders) No. 3, May 1982 and also from the U.S. Pat. No. 4,142,838. A method of the kind mentioned, in which the differentiation is realized by subtracting the delayed from the undelayed signal is also known from German Pat. No. 28 28 124. Pumping is understood to be an instable turbocompressor behavior in which the feed medium flows in surges or periodically from the compression or discharge side back to the suction side. This behavior sets in when the throughput is too low or the compression ratio between compressor inlet and outlet is too high. The so-called pumping limit line in the compressor's operating characteristic field separates the stable from the instable range. The pumping limit control insures that when the momentary working point approaches the pumping limit line or a blow-off line running parallel to the pumping limit line at a safety distance, a blow-off or recycle valve at the compressor outlet is opened. It is also known in this method to vary the control behavior of the controller generating the positioning signal for the blow-off valve as a function of the working point location by increasing non-linearly the control amplification when the blow-off line is crossed.
In such controls, the use of controllers with a control component differentiating the input signal encounters a number of difficulties. One of these difficulties is the fact that an actual signal derived from the throughput has a very high superposed noise level, thereby rendering a differentiation very difficult. This difficulty is less pronounced when the actual value signal is derived from the end pressure.
Another disadvantage of using a controller with a differential component is the fact that the characteristic constant speed lines and/or the characteristic field is formed by compression ratio and throughput. In the proximity of the pumping limit these compressor characteristic lines are very flat while they are steep far in the characteristic field. The consequence thereof is that the modification of the compressor end pressure near the pumping limit is only vert slight. Accordingly, the diffential component of the controller also has the least effect near the pumping limit. But this is precisely the characteristic field area in which a differential component of the controller would be needed the most to achieve a quick response, because the compressor is most endangered in this area.